The present invention is directed to a process for regenerating a used precious metal catalyst by redispersing the precious metal. Regeneration of used precious metal catalysts is an important aspect of this type of catalyst technology, intended to extend the lifetime of the precious metal catalyst. Regeneration is generally performed in three steps, namely 1) Removal of carbonaceous deposits (usually by burning), 2) redispersion of the precious metal, and 3) reduction of the redispersed precious metal.
Various methods for regeneration of used precious metal catalyst by redispersion of the precious metal on the support are known. Generally the redispersion is done by treatment with oxygen, Cl2 or Br2.
In a review article of J. B. Butt and E. E. Petersen in Activation, Deactivation and Poisoning of Catalyst, Academic Press (1988), pp 214 to 232, it has been indicated that the following general types of redispersion procedures have been suggested: 1) thermal treatment in an oxygen atmosphere, 2) thermal treatment in oxygen followed by reduction in hydrogen, 3) high temperature treatment followed by rapid cooling or 4) chemical dissolution of contaminants and redispersion of the metal. In this article it has been indicated that the behaviour of a catalyst in redispersion experiments is very much dependent on the nature of the support. A platinum on alumina catalyst can for example be redispersed using air, whereas platinum on silica behaves totally different.
In WO-A 95/23643 a process has been described for the catalytic treatment of waste water, using a precious metal catalyst, which process includes as part of the overall process the regenerating of the catalyst. The regeneration of he catalyst which is a palladium on silica/carbon, is done by washing with an organic solvent, and/or acid and/or by thermal treatment.
In U.S. Pat. No. 3,804,777 a process has been described, wherein lead contaminants present in a precious metal catalyst are dissolved by percolating a dilute acid at ambient temperature through the catalyst, followed by treatment with hydrogen at about 100xc2x0 C.
The present invention deals with a process for regenerating a precious metal catalyst on an amorphous silica alumina support. The inventors of the present invention have found that the conventional methods, for example such as reported by Butt et al, but also the methods using chlorine or bromine do not result in an increase of dispersion, as will be shown in the examples to be reported herein. Various known methods result in a decrease of the degree of dispersion.
It is an object of the present invention to provide a method for redispersing the precious metal present in a catalyst based on an amorphous silica-alumina support.
The process according to the present invention for the regeneration of a catalyst, said catalyst comprising at least one precious metal on an amorphous silica alumina support, comprises impregnating the catalyst with an acid, followed by reduction or oxidation of the impregnated catalyst at a temperature above 200xc2x0 C.
The regeneration of the catalyst, as used herein, indicates a redispersion of the said at least one precious metal over the surface of the support.
Surprisingly it has been found that the use of an acid in a liquid state, such as an aqueous solution, followed by the specific thermal treatment results in a redispersion of the precious metal on the support, whereas other methods result in a decrease of the degree of dispersion.
Suitable precious metals for use in the catalyst to be regenerated by the process according to the present invention are platinum, palladium, gold, iridium, rhenium, ruthenium, rhodium, osmium and silver. Also combinations of two or more of those precious metals can be used. So it is possible to use a combination of at least one of those precious metals with one or more other metals. The preferred catalyst to be regenerated by the process of the present invention is based on platinum and/or palladium.
The precious metal catalysts to be regenerated by the process of the present invention have generally been used for reactions involving hydrogenation, such as hydrogenation itself, hydro-isomerisation, hydro-desulfurisation and hydro-dewaxing. The may also have been used in dehydrogenation reactions, such as catalytic reforming. Once the performance, of the catalyst decreases below a certain level the catalyst is regenerated. In some processes it is also usual to perform regeneration simply after a certain amount of time has passed, without waiting for a decrease of the activity.
The catalyst to be regenerated is, either in the reactor or in a separate plant, prepared for the actual redispersion experiment. This step includes the removal, of carbonaceous deposits and other unwanted material on the catalyst. This can for example be done by washing using a suitable solvent and/or by burning of the contaminants.
Subsequently the catalyst is impregnated with an acid, preferably in an aqueous solution. Suitable acids are the usual mineral acids, including HCl, H3PO4, H2SO4, HNO3, HBr, or combinations of two or more of these acids. The amount of acid on the basis of the ratio of equivalents of acid to atoms of precious metal is between 0.1 and 100, preferably between 0.5 and 10. After impregnating the catalyst with the acid, the impregnated catalyst is either reduced in a flow of hydrogen gas or oxidised in a flow of dry air, followed by reduction. In a more preferred embodiment both reduction and oxidation are carried out at a temperature of at least 250xc2x0 C. and more in particular between 250xc2x0 C. and 600xc2x0 C.
After the final reduction step a supported catalyst is obtained usually having approximately the same degree of dispersion as the original catalyst. Sometimes the treatment even results in an increase of degree of dispersion. It is to be noted in this respect that the degree of dispersion is related to the precious metal crystallite size. The degree of dispersion can for example be determined by CO-chemisorption, whereby the amount of CO absorbed by the precious metal gives an indication of the number of metal atoms available on the surface of the metal crystallites. A larger amount of chemisorbed CO indicates a higher degree of dispersion, i.e. a smaller metal crystallite size.
An important aspect of the present invention resides therein that the catalyst to be regenerated is based on an amorphous silica alumina support. On the support a precious metal is present preferably in an amount of from 0.001 to 5 wt. %, calculated on the weight of the catalyst (in reduced form). The amorphous silica-alumina support is preferably prepared using a sol-gel method, whereas the support has an Si/Al atomic ratio of between 0.1 and 300. As has been indicated before, the type of support is extremely important in the selection of the regeneration method.
The present invention is now elucidated on the basis of a number of experiments.